One of Donald J. Trump’s campaign hats at an event in Boca Raton, Fla., on Sunday.Credit Eric Thayer for The New York Times

Donald J. Trump is famous for the nondisclosure agreements he requires from his employees. But even those volunteering for his campaign without pay are being asked to sign confidentiality agreements.

An email sent to prospective volunteers for a call center at Mr. Trump’s campaign headquarters in New York said that they would have to sign a nondisclosure agreement and submit to a “routine” background check.

Attendees were asked to be part of a briefing session. “We need to get you to sign an NDA, check your identification, and give you an idea of what you can expect as a volunteer in our call center,” read the email, which a recipient provided to The New York Times.

“Why are we doing a background check?” read a question on the email. The answer? “We need to make sure our call center is a safe environment for all of our volunteers and staff.”

Volunteers, according to the email, will be “fed” and given “frequent SWAG giveaways, and have special guests to drop by and say hello!”

Asked about the requests for privacy, a spokeswoman for Mr. Trump, Hope Hicks, replied by email: “This is standard practice and has been operational for months.”